The present invention relates to a process for manufacturing clad metal tubing by hot extrusion, in which one metal (or alloy) is clad to another metal (or alloy) having a deformation resistivity substantially different from that of the first one. Under usual conditions it is rather difficult to apply hot working, such as hot extrusion, to the combination of these different types of metals to produce a sound clad material. However, according to the present invention clad metal tubing can be obtained which is substantially free from surface defects and other defects.
Clad materials have been used widely in various applications. A clad material is a combination of two different types of metals (the term "metal" herein means both a pure metal and alloys thereof) in which desirable characteristics of each of the metals can be utilized.
Therefore, a variety of metals and combinations thereof are known in industry. The clad material produced in the largest amount is clad steel plate in which one of the metals (called the "parent metal") is carbon steel, low alloy steel, or the like and the other metal is stainless steel, titanium, or other corrosion resistant material.
Cladding has also been practiced in manufacturing many types of tubing. The most popular process for manufacturing seamless clad pipes is hot extrusion, e.g. the Ugine-Sejournet extrusion process, which is shown in FIG. 1.
In FIG. 1, blank pipes 1, 2 of different types of metals are combined to make a billet 3. The billet 3 is heated to a high temperature, and then subjected to hot extrusion. Manufacturing costs and properties of the product tubing are important considerations in determining the materials to be used for the blank pipes. For example, for use in line piping in which not only high strength but also improved resistance to corrosion are required, it is advantageous to use clad tubing comprising carbon steel or low alloy steel, which is less expensive and of high strength as the parent metal, and a nickel-base alloy with improved resistance to corrosion as the cladding layer. However, when clad tubing of this type is manufactured by conventional hot extrusion, a combined billet 3 is prepared by assembling a blank pipe 1 of carbon steel (or low alloy steel) and another blank pipe 2 of a nickel-based alloy. Usually, such hollow, thick-walled pipings are manufactured by a series of steps of melting, casting, forging, and machining (e.g. boring). The smaller one is inserted into the larger one to assemble a combined billet. After being heated to a predetermined temperature in a heating furnace and/or induction heating furnace, the combined billet is subjected to hot extrusion.
However, the hot extrusion of the prior art results in the following disadvantages.
1) Problems regarding surface characteristics of the product tubing:
One of the two metals, especially the one constituting the cladding layer, e.g., a nickel-base alloy in the case where carbon steel is clad with nickel-base alloy, is usually hard to work and the resulting cladding material suffers from various defects and cracking on the surface thereof.
2) Problems regarding bonding strength:
Bonding between the parent metal and the cladding metal is not perfect, and the strength therebetween is rather low. When the two metal layers are unbonded, hydrogen ions go into the space between the two layers to widen the space due to generation and expansion of hydrogen gas, resulting in swelling of the piping and a decrease in mechanical strength.
3) Problems regarding manufacturing costs:
Since many manufacturing steps are required until a combined billet is prepared, and the yield rate of product with respect to raw material is very small, manufacturing costs are very high. Carbon steel and low alloy steel are less expensive, and the efficiency of material thereof does not have any substantial effect on the manufacturing cost of the final product. However, the yield rate of the blank pipe of a nickel-base alloy which is very expensive has a great effect on the manufacturing cost of the final product. Furthermore, it is time-consuming to perform forging and machining of such a nickel-base alloy in order to manufacture a blank pipe, since it is very hard to apply forging and machining to the nickel-based alloy.
One of the solutions of problems 2 and 3 is to use metal powder as a starting material for manufacturing the blank pipe. For example, a wrought material is used to prepare a parent pipe of carbon steel or low alloy steel, and a powder material is used to prepare a cladding layer. Such powder metallurgical processes have been proposed in the following literature:
1 U.S. Pat. No. 3,753,704. PA1 2 U.S. Pat. No. 4,016,008 (Japanese Patent Publication 60-37162) PA1 3 Japanese Unexamined Patent Application Disclosure 61-190006 PA1 4 Japanese Unexamined Patent Application Disclosure 61-190007
According to the processes disclosed therein, as shown in FIG. 2, a combined billet is prepared, heated, and subjected to hot extrusion.
The combined billet shown in FIG. 2 is comprised of a hollow cylinder 1 (parent pipe) made of carbon steel or the like, a thin-walled metal pipe 5 (sometimes referred to as a "capsule"), and a powder-packed layer 4 provided between the hollow cylinder 1 and the thin-walled metal pipe 5. The upper and lower ends are sealed by end plates 6-1 and 6-2, respectively.
The thus-prepared billet is then heated to a predetermined temperature after the powder layer 4 is further packed by a cold isostatic pressing process or the like, if necessary. The heated billet is hot extruded to form clad tubing. During hot extrusion, the powder layer 4 is consolidated due to heating, compaction, and shear deformation to form a cladding alloy layer which is bonded to the inner surface of a parent layer comprising the deformed hollow cylinder 1. After deformation through hot extrusion, the end plates 6-1 and 6-2 and the thin-walled metallic pipe 5 are removed by pickling.
Usually, the hollow cylinder 1 is made of a relatively inexpensive and easily deformable material such as a carbon steel or low alloy steel. The powder-packed layer 4 is made of a powdery alloy which exhibits excellent resistance to corrosion. A typical such alloy is a nickel-base alloy. When powder is used, the yield of the product is almost 100% with respect to the starting material. This is very advantageous from an economic viewpoint.
FIG. 2 shows the case in which a cladding layer is provided in the inner surface layer of the pipe. The cladding layer may be placed in the outer surface layer of the pipe depending on the purpose for which the pipe is used. In that case, a capsule 5 is provided around the outer surface of the parent pipe 1, and powder is packed in an annular space between the capsule 5 and the parent pipe 1 to form a powder-packed layer 4.
It is to be noted that in this specification, the term "blank pipe" refers not only to a powder-packed layer in the form of a hollow cylinder which is formed by packing powder into a capsule, i.e., a thin-walled metal pipe but also to a wrought or machined hollow cylindrical metal. These two blank pipes may constitute a combined billet.
As is described in the above, when powdery metal is used to prepare a blank pipe, the bonding strength between the two blank pipes at the interface thereof is further improved in comparison with the case in which the two blank pipes are made of wrought metals. This is because upon hot extrusion particles which constitute metal powder bite into the surface of the other parent pipe to break down a thin oxide film. Thus, a fresh surface is formed to ensure reliable and improved bonding in comparison with the prior art cladding.
A hot extrusion process utilizing a combined billet in which a powder-packed layer is used as one of the blank pipes has been practiced only as a process for manufacturing carbon steel and stainless steel clad tubing. However, problem 1 mentioned earlier has not yet been solved.
Namely, when a hot extrusion process is applied to a combined billet which comprises a carbon steel parent pipe and a cladding outer shell of a nickel-base alloy, such as Alloy 825 or Alloy 625, a large, wavy deformation in wall thickness is produced, sometimes resulting in cracks resembling the shape of bamboo joints.
FIG. 15 schematically illustrates such cracks which occurs in a cladding layer having a tendency to be difficult to work. The parent base layer 17 is made of carbon steel which is easy to work and the cladding layer 18 which constitutes the inner layer of the tubing is made of a nickel-base alloy which is hard to work.
As shown in FIG. 15, although the thickness of the parent layer is somewhat irregular, there is a remarkable degree of nonuniformity in thickness of the cladding layer, which is hard to work. It can be seen that in places the cladding layer has been completely ruptured. These ruptured portions 19 are found at regular intervals in the longitudinal direction, similar to the joints of a piece of bamboo. Such defects, therefore, will be referred to as "joint-like cracks".
This type of defect cannot be remedied by subsequent handling or working, so the clad tubing would have to be scrapped if it occurs.
One of the causes of these joint-like cracks is that the resistance to deformation of a nickel-base alloy is high and the alloy is hard to work. Therefore, in order to eliminate joint-like cracks it seems to be helpful to heat the starting materials to a high temperature before working so as to decrease their resistance to deformation.
However, when the heating temperature of a billet is higher than the solidus line of the nickel alloy, intermetallic compounds are concentrated along crystal grain boundaries and a portion of the compounds may turn into a liquid phase. A degradation in the ease of pipe formation and the properties of the product is inevitable. Thus, increasing the heating temperature of a hard-to-work material is not a good way to solve the above-described problems of the prior art. In addition, it is impossible to completely remove the joint-like defects only by heating the starting materials to a high temperature. Thus, such an approach would result in nothing but energy loss.
As already mentioned, flaws and cracks in the surface of tubing require many steps to remedy. In particular, it is quite difficult and almost impossible to remove a flaw or crack from the inner surface of tubing, and if the flaw or crack can not be removed, the resulting tubing is of no value.